I think , this error is because your are returning a string and the class is expecting an instance of Authenticatable.
Create a middleware that checks if the username is a 'Slug'
Create () method when validation passes
clean up the AuthController by removing the slugCheck method
I suggest you , to follow this article for more information let me know if you face any problem again
https://www.cloudways.com/blog/laravel-paypal-payment-gateway/
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